# Homework Help: Cyclist coasting down a hill cons. of energy

1. Apr 21, 2013

### totallyclone

1. The problem statement, all variables and given/known data
A bicyclist coasts down a 7.0° hill at a steady speed of 5.0m/s. Assuming a total mass of 75kg (bicycle plus rider), what must be the cyclist's power output to climb the same hill at the same speed?

2. Relevant equations
EK=1/2 mv2
EG=mgh
P=W/T

3. The attempt at a solution
I tried the question and this is what I got but I'm answer if this is right... not sure if there's a Force of friction though because I read the question and assumed there isn't any force of friction... but I'm unsure... :uhh:

P=W/T
P=-WGΔdcos180°/T
P=-WGvcos180°
P=-mgsin7.0°(5.0)cos180°
P=-(75)(9.8)sin7.0°(5.0)cos180°
P=448W

2. Apr 21, 2013

### kontejnjer

Well if the cyclist goes down the hill with constant speed, then there must be some kind of force that's stopping him from accelerating, so you should account for it accordingly.

3. Apr 21, 2013

### tiny-tim

hi totallyclone!
he couldn't coast (at a steady speed) if there wasn't friction, could he?

4. Apr 21, 2013

### totallyclone

so.....THERE'S FRICTION!!?!??

5. Apr 21, 2013

yup!!!